Question

If the generic type argument (of either a calling class or calling method) is constrained with where T : Base the new method in T == Derived is not called, instead the method in Base is called.

Why is the type T ignored for method call even though it should be known before run time?

Update: BUT, when the constraint is using an interface like where T : IBase the method in Base class is called (not the method in interface, which is also impossible).

So that means the system actually is able to detect the types that far and go beyond the type constraint! Then why doesn’t it go beyond the type constraint in case of class-typed constraint?

Does that mean that the method in Base class that implements the interface has implicit override keyword for the method?

Test code:

public interface IBase
{
    void Method();
}

public class Base : IBase 
{
    public void Method()
    {

    }
}

public class Derived : Base
{
    public int i = 0;

    public new void Method()
    {
        i++;
    }
}

public class Generic<T>
    where T : Base
{
    public void CallMethod(T obj)
    {
        obj.Method();  //calls Base.Method()
    }

    public void CallMethod2<T2>(T2 obj)
        where T2 : T
    {
        obj.Method();  //calls Base.Method()
    }
}

public class GenericWithInterfaceConstraint<T>
    where T : IBase
{
    public void CallMethod(T obj)
    {
        obj.Method();  //calls Base.Method()
    }

    public void CallMethod2<T2>(T2 obj)
        where T2 : T
    {
        obj.Method();  //calls Base.Method()
    }
}

public class NonGeneric
{
    public void CallMethod(Derived obj)
    {
        obj.Method();  //calls Derived.Method()
    }

    public void CallMethod2<T>(T obj)
        where T : Base
    {
        obj.Method();  //calls Base.Method()
    }

    public void CallMethod3<T>(T obj)
        where T : IBase
    {
        obj.Method();  //calls Base.Method()
    }
}

public class NewMethod
{
    unsafe static void Main(string[] args)
    {
        Generic<Derived> genericObj = new Generic<Derived>();
        GenericWithInterfaceConstraint<Derived> genericObj2 = new GenericWithInterfaceConstraint<Derived>();
        NonGeneric nonGenericObj = new NonGeneric();
        Derived obj = new Derived();

        genericObj.CallMethod(obj);  //calls Base.Method()
        Console.WriteLine(obj.i);

        genericObj.CallMethod2(obj);  //calls Base.Method()
        Console.WriteLine(obj.i);

        genericObj2.CallMethod(obj);  //calls Base.Method()
        Console.WriteLine(obj.i);

        genericObj2.CallMethod2(obj);  //calls Base.Method()
        Console.WriteLine(obj.i);

        nonGenericObj.CallMethod(obj);  //calls Derived.Method()
        Console.WriteLine(obj.i);

        nonGenericObj.CallMethod2(obj);  //calls Base.Method()
        Console.WriteLine(obj.i);

        nonGenericObj.CallMethod3(obj);  //calls Base.Method()
        Console.WriteLine(obj.i);

        obj.Method();  //calls Derived.Method()
        Console.WriteLine(obj.i);
    }
}

Output:

0
0
0
0
1
1
1
2

Answer 1

Except when using dynamic objects, C# always binds methods at compile time–even when using generics. Virtual method calls are bound to virtual method slots rather than to the implementing methods, so that when they are performed on derived-class objects they will be directed to the derived-class implementations; although the methods to which the slots point will be determined at run time, the binding to the slots occurs at compile time. If a derived-class method is declared new rather than override, code which is bound using the derived class will use the derived-class method, but code which is bound using the base class will use the base-class method.

To understand why this has to be the case, imagine if it weren’t. What should happen if class Base declares a method int Foo(), and a class Derived:Base declares a new string Foo(). If a generic class with constraint T:Base tries to call method Foo on an object of type T, what should the return type of that method be?